Grease and finely divided suspended solids may be converted to floating matter by air or gas-aided flotation with or without flotation aids.
Aerated Skimming Tanks, Preaeration
A few decades ago separate aerated skimming tanks with 3 to 5 minutes detention time and 0.05 to 0.08 ft3 per gallon (0.375 to 0.60 m3 per 1000 liters) air supply were used to treat large volumes of waste with moderate grease content. Compressed air containing 1 to 1.5 mg per liter chlorine gas was often used to increase efficiency.
The current version of this treatment (preaeration, that is, aeration before primary treatment) accomplishes more than grease removal. It also facilitates sedimentation and helps to refresh septic waste, which combined with increased floating and suspended solid removal improves the BOD reduction.
For grease removal 5 to 15 minutes aeration, using 0.01 to 0.1 ft3 per gallon (0.075 to 0.75 m3 air per 100 liters) of air is usually sufficient.
Although aeration facilitates grease removal, alone it is not very effective. Therefore, for manufactured skimming units, more efficient, mechanized flotation processes are used.
Pressure and vacuum flotation techniques generate air bubbles by reducing the pressure of a supersaturated air-waste mixture or by applying vacuum to the mixture, which is saturated under atmospheric pressure. The liberated minute bubbles tend to form around and attach themself to suspended particles in the waste. With this type of equipment, capital and operating costs are both relatively high. Therefore this type of flotation unit is most popular for pretreatment of industrial wastes at the source, where the flowrate is less and the concentration is high.
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